Skip to main content

Full text of "The story of slate / by Charles H. Davis."

See other formats




By Charles H. Davis 

President, Davis Slate & Manufacturing Company 

Director t National Slate Association, Philadelphia 


The Story of Slate 

To understand slate properly one 
must know something of its origin 
and must be able to follow in some 
measure the slow but powerful 
geologic forces that have shaped it into its 
present form. 

The natural history of slate falls into two 
great periods, each period covering thousands 
and possibly millions of years. 

The first period has to do with the origin 
of the mineral particles of which slate is 

All rocks slowly wear away and disinte- 
grate through the action of frost and water. 
Sometimes fiction writers, orators or poets 
speak of the everlasting hills — but no hills 
are everlasting — the rocks that form them 
are slowly but continuously changing into 
new forms that are carried away and are 
built up into other rocks. 

Many minerals, particularly the feldspars, 
slowly change into clay and this clay is 
washed down by the rain, carried by streams 
into rivers, and by rivers to the sea. 

Clay particles are very small and light, so 
are carried far from shore and deposited in 
uniform beds on the ocean floor. 

Such clay beds are the raw materials from 
which slate is produced by the forces of 
nature. ___ 

The second great period comprises the cen- 
turies during which the unseen forces of 
nature convert the clay into slate as we now 
find it in the earth. 

The clay beds are first compressed into a 
firm mass known as shale. 

The shales may later be intensely folded 
and squeezed. 

It is generally assumed that the earth is 
slowly cooling, and as it cools, it shrinks. 

This shrinkage causes a wrinkling or fold- 
ing of the surface rocks just as the skin of an 
apple wrinkles when it is baked. 

When the shales are caught in these folds 
they are squeezed with terrific force and 
under high temperature. 

The forces are so great that in some 
mysterious manner the clay is converted into 

new minerals, the chief of which are mica 
and chlorite, both consisting of minute 
flakes and scales. 

Millions of these tiny flakes overlap each 
other like shingles on a roof. 

As they all lie in one plane there is de- 
veloped a remarkable tendency for the rock 
to split with great ease in one direction. 

This splitting tendency or cleavage is the 
most important property of slate. 

Slabs 4x6 feet in size, or larger, may be 
readily split to less than J^-inch thick, and 
there have been instances where sheets 8 x 16 
inches have been split to the remarkable 
thinness of -^-inch. 

The little tabular or flake-like grains are 
cemented under intense pressure which gives 
slate unusual strength. 

While human hands can take slate apart 
they can never put the grains together again 
with the same regularity, nor can they give 
it the same strength as is possessed by the 
natural rock. 

The fineness of grain, uniformity and 
strength of slate fit it peculiarly well for many 

Remember now that no rocks are everlast- 
ing and slate is no exception to this rule. 

It is noteworthy, however, that slate con- 
sists of non-metallic minerals that are very 
enduring and that resist remarkably well the 
forces of the weather that are ever at work 
in wearing down the rocks. 

Now what are some of the uses for which 
this remarkable product is best adapted? 

Its enduring properties and its tendency 
to split into thin sheets led to its early use for 
making weatherproof roofing for houses, and 
this is still one of its important uses. 

Black, gray, green, red, purple or mottled 
slates form many of the most beautiful roofs 
to be found in the world. 

It has also been found that high-grade 
slate is an excellent nonconductor of electric- 
ity, and as it can be readily shaped, drilled 
and polished, it is widely used for electrical 

Engineers in power-houses and factories 
take great pride in the attractive appearance 
of the large switchboards which form an 
important part of the equipment wherever 
electricity is generated or used. 

Slate slabs are attractive, and as they are 
easily matched in color a slate switchboard 
may be enlarged without detracting from its 

The roof is not the only part of a building in 
which slate is widely used. 

It may be seen in many buildings in the 
form of stair steps, baseboards, window sills, 
shower stalls, floor tile, etc. 

It is also fashioned into laundry tubs, 
water tanks, kitchen sinks, dough troughs in 
bakeries and similar furnishings. 

You are all familiar with slate in the form 
of blackboards, a use for which it excels all 
other materials. 

It is claimed that one area in Pennsylvania, 
26 miles long and 2 or 3 miles wide, provides 
most of the blackboard slate in the world. 

Our grandparents used school slates, but 
they have been largely replaced in the United 
States by exercise books, more commonly, 
and I hope incorrectly, called "scribbling 

School slates are still manufactured in 
great quantities, but about 90 percent of 
them are exported to foreign lands. 

You have now had a brief outline of the 
origin and history of slate and its chief uses. 

Let us turn to a natural ledge of slate rock 
as it lies in the mountain side, and follow 
each step in the process of shaping it into 
finished forms. 

Slate occurs in many parts of the country 
and is worked chiefly in the states of 
Pennsylvania, New York, Vermont, Maine, 
Maryland and Virginia. 

When a deposit of good slate is found the 
chief task is to remove all the soil or inferior 
rock from the surface leaving the good slate 

The slate may be removed by blasting in 
drill holes, but many operators now use 
channeling machines to cut out massive 

A channeler is a machine operated by 
steam or compressed air that chops the rock 
by repeated blows of heavy chisel-like bars. 

In this way a vertical channel or groove 2 
or 3 inches wide and possibly 12 feet deep is 
cut along the wall of the quarry, and cross 
channels subdivide the rock into large rec- 
tangular masses. 

The latter are broken free from the quarry 
floor by driving wedges in drill holes or by 
the discharge of a small amount of black 
blasting powder. 

The large masses are subdivided by split- 
ting along the direction of slaty cleavage and 
by making fractures in other directions with 
wedges driven into drill holes. 

Masses of slate thus obtained weighing 1 
to 3 tons are hoisted to the surface. 

Great chains are placed about them and 
they are elevated with steel cables wound on 
drums by powerful hoist engines. 

Some slate quarries have been worked for 
many years and thus are of wide extent and 
very deep. 

Several of the Pennsylvania quarries are 
more than 400 feet in depth. 

The treatment that a block of slate receives 
after it is removed from the quarry depends 
entirely on the purpose for which it is to be 

If it is to be converted into roofing slates 
the process is very simple. 

The block is first subdivided into masses 
24 to 30 inches in length and the thickness of 
8 slates. 

This is the task of the block-maker. 

The splitter then takes these masses and 
with a thin flexible steel chisel and a wooden 
mallet he subdivides each block, always 
splitting it in the center, until the 8 slates 
are obtained. 

The trimmer then places them on a trim- 
ming machine where the irregular edges are 
cut away leaving the largest perfect rectangle 
of a given standard size that the slab will 

Trimming is done with a heavy blade like a 
great meat cleaver, operated with a foot 
treadle, or sometimes with a curved rotating 
blade like that of a lawn mower. 

The finished slates are piled in racks 
according to size, and when holes are punched 
in them for nailing they are ready to be placed 
on a roof. 

Slate used for other purposes is usually 
termed "structural slate" or " milled stock." 

Mills equipped with various machines are 
required for the manufacture of structural 
slate products. 

The first milling process is most surprising. 

You have all at some time or other ob- 
served a circular saw cutting logs into lumber 
or boards into shorter pieces. 

You may have wondered at the ease with 
which a circular saw eats its way through a 
hard piece of timber, but it seems even more 
remarkable that a circular saw can be used 
to cut a piece of rock. 

Such is the case, however, for the first 
operation in the mill is to place the block of 
slate on a heavy traveling bed that carries 
it against the teeth of an especially designed 
circular saw. 

The bed travels very slowly, and the saw 
rotates at much slower speed than a wood 
saw, but a mass of slate 1 foot thick and 4 
feet wide may be cut across in a very few 

The blocks obtained may be split to the 
desired thickness for blackboards. 

If structural slabs are desired the blocks 
are placed on a second traveling bed and 
passed repeatedly beneath a heavy blade 
which scrapes the surface smooth. This ma- 
chine is called a "planer." 

The surfaces may be sand-rubbed and 
polished by other machines. 

Edges may be trimmed or bevels cut with 
carborundum wheels. 

In a properly arranged slate mill the rough 
block enters at one end and passes from one 
machine to another in a regular order until 
it is prepared for shipment. 

An interesting feature of slate-working is the 
importance of keeping the block moist until 
final splitting is accomplished. 

As the block lies in its natural bed it con- 
tains what is called "quarry water." 

If this water once dries out, the slate will not 
split readily, and no amount of wetting will 
renew the splitting property if once it is lost. 

All quarry blocks are conveyed directly to 
the mill, and if there is any long delay water 
is thrown over them or they are covered to 
prevent evaporation. 

Only the highest quality of slate is used 
for the manufacture of slate products, and 
all defective blocks are thrown away. 

Thus great mountains of waste are built up 
around slate quarries, and one of the big 
problems in the industry is to find uses for 
this waste material. 

Some of it is ground to a fine dust and 
mixed with asphalt for making roads. It is 
also used in such products as floor linoleum, 
rubber and paint. 

From the brief historical outline presented 
you will appreciate that slate is a remarkable 
material which nature has endowed with 
peculiar and valuable properties. 

This short description of uses and proc- 
esses shows you that man has invented 
methods whereby these unusual properties 
may be turned to practical use with the result 
that slate furnishes many useful products 
which contribute to comfort and convenience. 

Sometimes on foggy mornings objects appear 
with dim outlines, and with difficulty we 
distinguish a house from a tree. 

We see, but our vision is not clear. 

On the other hand we sometimes see ob- 
jects plainly in so far as our actual eyesight 
is concerned, but the object conveys little 
impression to our minds because it is not 
understood — our lack of information about 
the object covers it like a fog, and we see 
only its outlines. 

Possibly with most of you slate is one of 
those objects which you see with the eye 
only, and this short story will have served its 
purpose if it clears away the fog in some 
measure and enables you, when you see slate 
with the eye, to see it also with the under- 

June i, 1923